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Underwater Robotic Fish
Phase II: Buoyancy P15029
Fred Cookhouse
Sarah Bailey
Chloe Bohlman
Igor Drobnjak
Brandon Micale
Mark Pitonyak
Mechanical Engineer
Mechanical Engineer
Electrical Engineer
Mechanical Engineer
Biomedical Engineer
Electrical Engineer
Project Summary
The objective of this project is to
create an underwater robot that
looks and swims like a fish. The
fish is to achieve these biomimetics
by utilizing McKibben muscles to
hydraulically propel the fish. The
fish will be able to swim forward
and turn, and have depth control
using active buoyancy in both R/C
and autonomous modes.
This project is a continuation of
P14029, who built a robotic fish
capable of swimming forward and
turning. The project was an
exercise in biomimicry and used
artificial McKibben muscles to
propel the robot.
From left to right: Ballast Tank: At neutral buoyancy, submerging, and surfacing.
Buoyancy System
Original CAD Model of Final Design Phase MSD I
Goals and Objectives
•
•
•
•
Design an active buoyancy system
Add underwater remote controlling
Program an autonomous swim mode
Improve physical aesthetics of the fish
The original fish was able to swim only at the surface of the water and subsequently,
the customer wanted to include active buoyancy control in this second phase, with
the ability to go up and down within a 3ft depth. The 3ft depth was quantified by the
depth of the swimming pool available for demonstrations. With this requirement, it
was essential to understand the physics behind buoyancy.
Buoyancy is the force exerted on an object that is immersed in a fluid. Essentially,
the buoyant force on an object is equal to the weight of the fluid it displaces. If the
buoyancy force is less than the weight of the object, then the object will sink, called
negative buoyancy. If the buoyancy force is greater than the weight of the object,
then the object will float, called positive buoyancy. To achieve neutral buoyancy, the
buoyant force and weight of the object must be equal.
This system is modeled after a ballast tank. The air bubble originally present in the
PVC container begins to compress as more and more water fills in. The compressed
air and added water increases the weight for the same volume of space, thus forcing
the fish to sink in the water. Once released, the compressed air pushes the water
back out of the system bringing the fish back to the surface.
Actuated McKibben style muscle (top), and relaxed McKibben muscle (bottom)
McKibben Style Muscles and Swimming
McKibben Style Muscles contract when the tubing expands into its braided sleeving
when pressurized. McKibben Style Muscles historically have been used pneumatically
– using pressurized air for contraction. However, for the Underwater Robotic Fish, the
McKibben Style Muscles are utilized hydraulically, where pressurized water contracts
the air muscles. The ends of the McKibben Style Muscles are attached via tensioning
cables to joints in the tail. When actuated the muscles pull on those tensioning cables,
moving the joints, and producing a swimming motion. The McKibben Style Muscles
and tensioning system allow the fish to use its surroundings as a means of propulsion.
Wireless Communication
Eco-Flex Skin
Communicating wirelessly to the fish
underwater
presented
some
unique
challenges. Typical close range wireless systems
like Wi-Fi and Bluetooth use a transmission
frequency of 2.4 GHz. This high of frequency
works well in air but is strongly attenuated in
water. Our solution was to develop a lower
frequency transceiver that operated at 315 MHz.
This design aptly accommodated the close range
and shallow depth requirements set for the
project. To maintain a communication link over
farther underwater distances, a more robust
system using acoustic transmission would be
appropriate.
On-Off Keying (OOK) Modulation
Eco-Flex are platinum-catalyzed silicones that are versatile and extremely easy to
mold and use. To produce a more life-like robotic fish, an Eco-Flex sheet was
molded to wrap around the tail. The Eco-Flex stretches, moves, and feels like flesh.
Over the tail, the Eco-Flex produces the desired aesthetics of a fish.
Oscilloscope signals for transmitted
information (top) and received data (bottom)
Acknowledgements
315 Mhz RF Module
Special Thanks
Dr. Kathleen Lamkin-Kennard and Rick Lux
Mark Schiesser and Triline Automation
Dr. Gomes, Dr. Sciremammano, and Dr. Schrlau
Previous Team – P14029